Acute lung injury is a major cause of respiratory failure in critically ill patients, and it is characterized by a neutrophil-dominated inflammatory response. Inflammation damages the airway epithelium and supporting connective tissue, leading to microvascular leak and then lung tissue fibrosis, the clinical hallmarks of the disorder. Thus, regulation of interactions between neutrophils and epithelial cells likely control fundamental mechanisms of tissue repair, and disrupted regulation may lead to lung injury. Recent studies demonstrate that neutrophil recruitment in a model of acute lung injury depends critically on matrix metalloproteinases (MMPs). MMPs recruit neutrophils by releasing potent chemoattractants from epithelial cells in vitro and in lung injury models in vivo. Neutrophils are a major source of reactive intermediates, and oxidants are potent activators of MMPs in vitro. Also, many lines of evidence suggest that oxidative stress increases when humans develop acute lung injury. These observations indicate that reactive intermediates may contribute to neutrophil recruitment, MMP activation, and the pathogenesis of lung injury by a variety of mechanisms. The overall goal of this proposal is to test the hypothesis that reactive species generated by inflammatory cells of the lung are critical for regulating MMP activity and mediating acute lung injury. We will approach this issue by investigating MMP oxidation and activation in vitro, analyzing MMPs in human lung samples for evidence of oxidative damage, and using mouse models to investigate oxidative damage and MMP activation in acute lung injury. These complementary studies will enable us to determine whether mechanisms of MMP activation that we uncover in vitro also operate in vivo. [unreadable] [unreadable]